Do fish sleep?

The short answer is yes; most fish do sleep. While most fish don’t have eyelids, they do have a regular period of reduced activity and metabolism. This does not exactly serve the same purpose as a good night’s sleep for you, but research has shown that fish sleep does serve a restorative function, saving energy and refreshing memory circuits in the brain. Though they are in a restful state, many fish are still acutely alert for danger – sleeping, in essence, with one eye open. Others, like the Spanish Hogfish (Bodianus rufus), sleep so soundly, that they can be lifted all the way to the surface before eliciting a response. Some species of parrotfish actually make their own “mosquito net” out of mucus to protect themselves from parasites so that they can sleep in peace.

Sleep may have evolved as a means to “reset” brain circuits through phases of unresponsiveness to sensory inputs. Organisms with brains that are capable of processing massive amounts of sensory information also need to be able to refresh memory circuits for infrequently used functions. Sleep serves as a time for fish to disengage from their surroundings and gives them a time for circuit refreshment.

Some fish don’t sleep.

Fish that swim continuously and blind cave fish do not sleep in a traditional sense. Researchers hypothesize that these types of fish may have less need to process sensory information, particularly visual information, and, as a result, their brain does not need to rest and reset in the same way that other fish do. For example, sharks and tunas that are obligate ram ventilators must swim to breathe so they cannot rest or sleep. These sharks and tunas also live in open water pelagic habitats, with very few visual features to register. Schooling fish, similarly, require less sensory processing because most fish are inside the school and rely upon directional information from other individuals. Blind cave fish, by virtue of being blind, do not register visual information at all.

Fish can have sleeping disorders, too.

Fish can be insomniacs; well, if researchers modify their biology. Zebrafish (Danio rerio) have been used to study sleep disorders. Researchers monitored the sleep patterns of normal Zebrafish and those that lacked a functional hypocretin brain receptor (which has been linked to narcolepsy in other organisms). The mutant fish fell asleep 30 percent less often than the normal fish and when they did finally catch some zzz’s, they could only sleep half as long as normal fish. Another study with Zebrafish showed that ambient light is very important to sleep duration. But, that sleep deprivation isn’t as stressful to a Zebrafish as, say, a college student during finals week. So, don’t get too worried about getting your pet fish an energy drink to make it through the day!

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While doing night surveys in clear and/or shallow water, I’ve often seen centrarchids hanging motionless and head-down in stumps, vegetation, nests, or other structure. If you’re careful, you can net them right up before they wake up.

The idea of fish sleeping is interesting indeed. But, have you ever seen a fish yawn? I was cleaning the aquariums at Inland Seas Education Center last week when I happened to look over at one of the aquariums an saw a smallmouth Bass open it’s mouth real wide and at the same time open it’s gills extremely wide as if yawning. I must have really been boring to him.